MPEG2 (Motion Picture Coding Expert Group Phase 2, ISO/IEC 13818-2) is standardized and widely used as a coding system for coding a motion image to record it on the digital recording media. The coding system is utilized for recording information and in addition, utilized in the fields of broadcasting and communication such as digital broadcasting and digital transmission.
The motion image compression coding system represented by the MPEG2 is a system combining intra-frame coding and inter-frame coding employing motion prediction, and compression coding by the variable-length code.
According to the motion image compression coding system, data are formed of an intra-frame coded frame coded within a frame and an inter-frame coded frame coded by referring to another frame to compress data.
The intra-frame coded frame is referred to as I-Picture (Intra-Picture). The inter-frame coded frame includes P-Picture (Predictive-Picture) compressed by referring to a frame in the past and B-Picture (Bidirectional-Picture) compressed by referring to two frames respectively in the past and in the future.
The P-Picture is coded and decoded by referring to a reference frame (I-Picture or P-Picture that is a frame referred to) which is present in the nearest past in order of reproduction time, and B-Picture is coded and decoded by referring to a reference frame present in the nearest past and a reference frame present in the nearest future.
FIG. 16 briefly shows a reference relation between frames according to the system of compression coding for a digital motion image, implemented by combining the intra-frame coding and the inter-frame coding represented by MPEG2 (ISO/IEC 13818-2).
In the drawing, reference numerals 101-110 all indicate frames that are sequential in time. Frames 101 and 110 denoted by I in the drawing are referred to as I-picture according to MPEG2, that are intra-frame coded frames generated according to the intra-frame coding. These frames are compressed by a system that allows only the image data coded within the frame to be decoded, that is, the frame can be decoded individually.
Those frames denoted by P and B in the drawing are inter-frame coded frames compressed by referring to other frames and using a difference of data obtained by motion compensation.
The drawing illustrates that a frame indicated by the head of the arrow refers to the frame at which the starting point of the arrow locates for determining motion and difference in order to achieve compression. According to MPEG2, a frame referring to only a past frame is referred to as P-Picture, and a frame referring to image data in the past and future is referred to as B-Picture. Frames 104 and 107 indicated by P in the drawing are P-Pictures, and frames 102, 103, 105, 106, 108 and 109 indicated by B in the drawing are B-Pictures. For P-Picture, compression is performed by using P-Picture or I-Picture present in the nearest past in terms of time as a reference frame, and for B-Picture, compression is performed by referring to P-Picture or I-Picture in the past and future.
Therefore, all of the frames refer to frame 101 directly or in a succeeding manner until frame 110 is presented that is I-Picture to be intra-frame coded next. Accordingly, frames 102-109 are compressed. In view of this, decoding of these P-Pictures and B-Pictures is impossible unless I-Picture to be intra-frame coded is initially decoded. A plurality of frames 101-109 in such a reference relation are all together referred to as GOP (Group of Pictures).
Further, the MPEG2 system employs the variable-length code. Accordingly, the quantity of data of each frame is not predetermined. For example, just an average quantity of data per unit time may satisfy a prescribed value in a long period such as GOP period formed of a plurality of frames, a sequence period formed of one or a plurality of GOP(s), or one program period.
FIG. 18 is presented for describing change of the quantity of data for every frame. Generally, a relation is established in the data quantity represented by I-Picture&gt;P-Picture&gt;B-Picture. However, the data quantity is not constant even if the frames are coded according to the same system.
In addition to the full-color motion image compression standard MPEG2 with its standardization promoted by ISO, MPEG (Motion Picture Coding Expert Group, ISO 11172-2) and H:261 series which is the standard of TV phone represent such compression coding scheme.
FIG. 17 illustrates a structure of a motion image recording apparatus that records an image data train coded according to this coding system on a recording medium.
Various recording media available for digital recording such as the optical disk, magnetic disc and magnetic tape can be used as a recording medium 15.
A recording conversion circuit 13 performs processing necessary for recording an input data train on the recording medium, such as blocking, addition of any error-correcting code, and modulation. Data thus processed are output as block data of a record unit of recording medium 15.
A recording control circuit 16 is a control circuit performing any control for recording an image data train on recording medium 15.
An input image code train is converted by recording conversion circuit 13 to any form appropriate for recording, and thereafter used as data block of a record unit to be recorded unit by unit of recording on recording medium 15.
Where the image code train is recorded for each record unit, it is impossible to specify the position at which the leading end of a frame in the image code train on the recording medium is located in the record unit of the recording medium (see FIG. 18).
FIG. 19 illustrates a specific example of a motion image reproduction apparatus for reproduction of a recording medium on which a data train is recorded as described above.
A reproduction control circuit 25 is a control circuit that directly controls reading from a recording medium 15. A reproduction conversion circuit 21 is a circuit that performs processing corresponding to the processing by recording conversion circuit 13, that is, demodulation, error correction, cancellation of blocking, in writing for recording medium 15.
A header detection circuit 22 is a circuit for searching the head and tail of I-Picture, for example, in order to reproduce only a reference frame from the data train read from recording medium 15, in any special reproduction such as high-speed reproduction.
In normal reproduction, the block data read film recording medium 15 unit by unit of record are converted back to a reproduction image code train by reproduction conversion circuit 21 and provided as a continuous image code train by a buffer memory 23. The image code train is reproduced as an image signal by a decoding circuit 24 to be provided as a reproduction image.
The high speed reproduction can be achieved by, for example, reproducing only those intra-frame coded frames in turn that can be reproduced without depending on any other frames.
FIG. 20 illustrates a relation of frames that are reproduced in the high speed reproduction.
In this drawing, I denotes I-Picture, P denotes P-Picture, and B denotes B-Picture. The time passes from left to right in the drawing. This drawing represents that the frame indicated by the head of the arrow is reproduced after the frame located at the starting point of the arrow is reproduced. The high speed reproduction is easily achieved by reproducing only the image code train corresponding to the intra-frame coded frame.
Alternatively, another type of high speed reproduction can be achieved by reproducing I-Picture which is an intra-frame coded frame and P-Picture which is an inter-frame coded frame in turn as shown in FIG. 21. The reason is that P-Picture is a frame referring to another frame and can be decoded by initially reproducing a reference frame present before the P-Picture.
Further, high speed reverse reproduction can be implemented by reproducing only I-Pictures in the reverse direction as shown in FIG. 21. The drawing represents that the frame located at the starting point of the arrow is first reproduced and then the frame indicated by the head of the arrow is reproduced.
Header detection circuit 22 finds the head and tail of the frame by reading a header code located at the leading end of each frame of the image code train. Header detection circuit 22 further determines a frame coding type shown by the header code (whether the frame is I-Picture, P-Picture or B-Picture) to detect a frame to be used for the special reproduction.
Header detection circuit 22 instructs buffer memory 23 to write an image code train of a frame upon finding the leading end of the frame to be used for the special reproduction, and then stops writing operation of buffer memory 23 upon detecting the tail thereof. Header detection circuit 22 thereafter searches a leading end of an image code train to be next reproduced. This operation is repeated.
Decoding circuit 24 reads an image code train from buffer memory 23 and outputs it as a reproduction image. The special reproduction is thus implemented.
However, according to a conventional technique, conversion by reproduction conversion circuit 21 and determination by header detection circuit 22 as to which frame is I-Picture or P-Picture are required in order to search a leading end of each frame in the high speed reproduction. In other words, the conventional technique has a disadvantage of the complicate frame retrieving process in the special reproduction. Accordingly, a remarkably high speed reading apparatus is required. In addition, if the reading speed is not enough, the retrieving speed cannot fully follow the reproduction speed to cause any imperfection in a reproduction image, or interrupt the special reproduction to be completed.
This problem is more serious in the reverse reproduction since the reading speed should be higher than that for the high speed reproduction.
Japanese Patent Laying-Open No. 6-46368 discloses the technique of matching data with a record unit for every GOP period in order to easily read motion image data. However, it is impossible to match a leading end of a specific frame such as I-Picture and P-Picture with a record unit.
The present invention is made to solve such problems as described above. The first object of the present invention is to provide a reproduction apparatus that can efficiency reproduce only a specific frame from an image data train coded frame by frame according to intra-frame coding and inter-frame coding.
The second object of the present invention is to provide a recording apparatus that can efficiency record a motion image on a recording medium for such a reproduction apparatus.